Genetic mutations causing human disease are conventionally thought to be inherited from one's parents and present in all somatic (body) cells. Increasingly however, somatic mutations are implicated in neurological diseases. Somatic mutations that arise during the cell divisions of prenatal brain development are inherited in clonal fashion and can cause neurodevelopmental diseases such as epilepsy and intellectual disability, even when present at low levels of mosaicism. New, highly sensitive technologies that we have employed in published work allow the first systematic analysis of clonal somatic mutations in normal brain development. Analysis of developmental, clonal somatic mutations would allow the first systematic analysis of somatic mutation rates in brain, and the first systematic picture of the pattern of cell divisions that generates the human brain. In this project we will apply our existing to provide several scientific discoveries not otherwise attainable, providing tools of widespread utility to the genetics and neuroscience communities. In this study, our three Specific Aims will be to 1) identify and catalogue the mutations which shape the somatic neuronal genome; 2) perform a cell lineage analysis of the adult human brain using clonal somatic mutations in cortical neurons; and 3) relate cell lineage patterns to cell phenotype in the human brain by separating neuronal, glial, and other cell types. These data provide three major discoveries which have all been major goals of neuroscience and which are not presently obtainable by other means: 1] the first determination of rates of somatic mutation in human brain, 2] the first direct cell lineage data from the adult human brain, which is essential for understanding how our brain develops and how developmental somatic mutations cause disease, and 3] a preliminary lineage map connecting neuronal cell classes.